I have put 2 radiators in my 98 jeep grand cheroke since I got it a year ago and now I need to replace it again, I just replaced this radiator about 2 weeks ago and it is leaking again.

So I did some searching and it seems like electrolysis is a big factor in radiator failure and I did the test which is using a multimeter - connect the negative lead to the negative battery cable, remove the radiator cap and put the posrtive lead of the meter in the coolant (making sure not to touch the radiator) set the meter to DC voltage and if you get a voltage reading higher then 0.10 volts, then there is current running through the cooling system.

I did this and get a reading of -0.28 volts, thats right negative voltage and yes I have the leads connected right if I touch the positive battery terminal with the lead I get good battery voltage (not negative). so what does this mean. I assume it means I have current draw int he cooling system and how do I figure out where the current is coming from. I did this test with the engine off and the key off, so there was now power draw by any component.

What brand coolant do you use? I'd seriously consider using distilled water when you fill the radiator with a water/coolant blend, but two weeks sounds like a manufacturing defect, are they OEM? After market radiators can be pretty bad.
Is the radiator mounted on rubber insulators?
I'm not too sure that a few tenths of a volt means anything - you can get voltage from a lemon. You can get ph strips to measure the acidity or alkalinity of the coolant.

I always used PRESTONE ALL MAKES ALL MODELS and mixed it myself to save money. But I used PRO SERVICE Antifreeze - which is what we use at work it is a universal coolant.

I just think there has to be something wrong with my jeeps. This would be the third radiator I put in this jeep - in a year. and I put 3 radiators in my old ZJ in about 3 years.

I always used PRESTONE ALL MAKES ALL MODELS and mixed it myself to save money. But I used PRO SERVICE Antifreeze - which is what we use at work it is a universal coolant.
I would avoid those at all costs - coolants have undergone major changes in the past few years and Prestone is no exception. The Prestone green is not the same Prestone that we have been using years past.
In fact, I have a document somewhere [Edit: it's in the '02 FSM and in MACS.] from Chrysler with a warning not to use it, or any other universal coolant. They recommend their Chrysler branded long life coolant which is HOAT, the same as Zerex G-05, or, Zerex Original Green which is IAT.
I use the latter in my Grand and have had zero corrosion problems.

This is the coolant I recommend for your Grand, it's the original IAT green formulation that came with your (and my) Grand and Valvoline is the only manufacturer that still makes it.

http://www.valvoline.com/pages/products/product_detail.asp?product=8

Also, NEVER mix brand/types of coolant, that guarantees problems.

I'm still curious if you're only using after market radiators, that could be a big part of the problem.

the first many radiators I used were all from RADIATOR EXPRESS. but this last one I just got (2 weeks ago). Was a VISTEON brand radiator from one of my local shops, I have installed many of these with very good luck in customers vehicles.

Where can I buy that ZEREX Orignial Green Antifreeze locally not online?

Where can I buy that ZEREX Orignial Green Antifreeze locally not online?
Try Napa, that's where I found it in my area.

http://royalcrossfarm.com/electrolysis.htm.
For electrolysis to effect a radiator that quick there would have to be more voltage than that, alot more. Have a read.

Well I did the test on a friends jeep. And I actually got 0.68 volts, from his system. Where I got a -0.28 from mine. And his cooling system is just fine no leaks at all. So I guess I will have them send over a new radiator. I haven't replaced it yet because I wanted to get the problem fixed first, but it looks like I just got a dud.

I would seriously consider buying an OEM radiator this time, you can get a good discount on the web.

Buy better radiators..You are just getting poor quality. Most radiators are being make overseas now days. Any radiator built correctly would last longer than that even if you put in pure water. I would look at it from the quality point. Do not use the "NEW" fits all antifreeze... The OEM's use specific anti-freeze for a reason. Go to your local dealer and buy it its not that expensive.

"You have a big investment in your vehicles..why would you repair them with the cheapest thing you can find....." (just my opinion)

well the radiator I bought was only $15 cheaper then the one from the dealer but the dealer had to special order this radiator and would take up to 10 days to get. I couldn't wait that long. And I have installed 2 dozen of these (visteon brand) radiators in customers vehicles and have not had one complaint in the last 2 years of working at the shop. I don't think it is the quality of the radiator I am thinking it is more of a problem inside my jeep. And the radiator I bought was an OEM fit radiator but not MOPAR BRAND.

Also, I don't think the brand of anti-freeze is going to cause a problem that quick..

Also, I replaced the Radiator again on Wesnesday and this morning the radiator is leaking again. I am going to get it to the RAD SHOP 2morow, hopefully, for them to run some test and see what they can come up with.

While on the topic of radiators ciming in from overseas, make sure you ask and SEE the manuf. labeling of any radiators, even the ones being sold by the dealers.

Guess where some of them are coming from, it may not be happening at the Jeep's parts dept. nationwide, but I know for other US car makers, their radiators were not made in the States.